(a) Field of the Invention
The present invention relates to a new photosensitive resin composition controlling solubility by hardness control and a pattern formation method of a double-layer structure using the same, and more particularly to a photosensitive resin composition that differentiates film thickness during developing of a photosensitive solution in an LCD (liquid crystal display) manufacturing process, which is useful for a double-layer structured color filter and overcoating material.
(b) Description of the Related Art
The conventional methods of forming color filters of liquid crystal displays are a dye method, a print method, an electrodeposition method, an ink-jet method, and a pigment dispersion method. Recently, the pigment dispersion method, which is advantageous in terms of pattern resolution and manufacturing, has been adopted. This method is applied for manufacturing LCDs of mobile phones, notebook computers, monitors, and TVs.
Now, pigment-dispersed compositions having more improved capabilities, not to mention the pattern resolution, are required. The general pigment-dispersed composition forms colored pixels by coating, exposing, and developing the substrate. In this case, hardening of the exposed pixels makes them insoluble in the developing solution. For this reason, substantial exposure energy is required, and if the exposure energy is insufficient, the light (UV) does not fully reach the lower layer of the coating film. Therefore, the hardness of the lower layer becomes weak and pattern breakup occurs. Also, if the coating solution contains a lot of pigments or if the light blocking is excessive, the pattern breakup becomes intense and much higher exposure energy is required.
If the exposure energy is high, the developing solution does not dissolve the surface of the coating film at all, and the film thickness does not change after developing. On the other hand, if the exposure energy is low, the hardness of the lower layer becomes much lower, which decreases adhesion to the substrate and causes pattern breakup.
In general, the conventional photosensitive resin composition comprises a) a binder soluble in an alkaline solution; b) a crosslinking monomer having at least two ethylene double bonds; c) a pigment; d) a photopolymerization initiator; and e) a solvent. If necessary, it may include additives such as an enhancer for improving adhesion to the substrate, a stabilizer for improving storage stability, and a disperser for improving dispersion of pigments. Generally, this photosensitive resin composition reduces solubility to the developing solution during the developing process, by forming crosslinkages with crosslinking monomers due to radicals of photopolymerization initiators generated by the light. Here, the light blocking due to pigments causes differences in hardness of the upper layer and lower layer. That is, the hardness is lower at the lower layer. This can be the cause of film breakup or undercut by the reaction of the lower layer with the developing solution, if the exposure energy is low.
With variation of small-to-medium-sized device characteristics, differentiated characteristics are required. Formerly, with regard to optimization of the EMB (embossing) process of reflectors for maximizing reflection efficiency of the reflection window, there was an attempt to solve the poor display characteristics in the portion in which there is no ambient light, which is a disadvantage of the reflective LCD, through a front-light, transflective LCD. The recent trend seems to follow the transflective LCD techniques, and the improvement of color filters in this regard is diversely in progress.
Among these, there is an attempt to expand the transmission window portion of the TFT-LCD, and thereby to increase color reproduction of the transmission window on the color filter and maximize reflectivity (transmissivity) rather than color reproduction of the reflective light.
Among these, with regard to change in chromaticity of the reflection window and the transmission window, there is an attempt to secure two tones on the color filter. Generally, securing two tones on the color filter means that the color filter characteristics of the transmission window and the reflection window are divided. With regard to difference of color characteristics of the reflection window and the transmission window, the basic problem is that the path of one is two times that of the other due to the difference of transreflection mode. In order to solve this problem, there are attempts to equalize optical properties of the transmission window and the reflection window by changing the optical property of the reflection window. Among these, there are the Toray method, which drills a hole on the reflection window and additively mixes light passing through the hole and the p.r region (FIGS. 2a, 2b, and 2c), and the Seiko-Epson method which differentiates chromaticity of the transmission window and the reflection window using a change in thickness of the transparent organic insulation film (FIG. 3).
The Toray method secures a chromaticity difference by following the process represented by FIGS. 2a, 2b, and 2c to cause a color difference of the transmission window and the reflection window. However, if the hole size is enlarged to maximize optical efficiency of the reflection window, the following process is affected, which is a problem with respect to controlling liquid crystal around the hole area. In applying an overcoat process to flatten the hole area, if the CF pattern is larger than 1.0 μm, the display characteristics becomes unstable due to the instability of the PI print and rubbing processes.
On the other hand, the Seiko-Epson method secures a chromaticity difference by applying the structure of FIG. 3 to cause a color difference of the transmission window and the reflection window. However, an increase of the thickness of the transparent film is necessary to increase the color difference between the reflection window and the transmission window, which causes problems of cell gap and orientation of the liquid crystal.